Abstract

Samples of marble xenoliths, thermally metamorphosed by the igneous rocks of Mont Saint-Hilaire, Quebec, contain idiomorphic plates of phlogopite. These plates were later overgrown and to some extent topotactically replaced by sodian tainiolite, tapering in fragile fibers extending parallel to [100]. The fibers only protrude parallel to [100] of the phlogopite host. Unit-cell parameters of phlogopite and tainiolite, both 1M polytypes (space group C2/m), are so similar that the phases in the phlogopite –tainiolite intergrowth could not be distinguished by single-crystal X-ray methods. Phlogopite of composition K1.01(Mg1.96Fe0.66Li0.20Mn0.03Ti0.03Al0.04)∑2.92(Si3.28Al0.72)∑4O10[(OH)1.14F0.86]∑2 has the highest Si/Al value of natural samples of phlogopite known to us. The epitactic fibers of tainiolite, of composition K1.01(Mg2.00Fe0.01Li0.59Na0.38)∑2.98(Si3.99Al0.01)∑4O10[F1.79Cl0.01 (OH)0.20]∑2, represent to our knowledge the first example of an extended solid-solution, established by combined electron-microprobe and laser-ablation mass-spectrometry analyses, between tainiolite, K(LiMg2)Si4O10F2, and the recently described mineral shirokshinite, K(NaMg2)Si4O10F2. The latest mineral formed in the observed paragenesis is the zeolite mesolite, accompanied by an unidentified aluminosilicate with Ca/K ≈ 1.